It is often difficult to acquire a global positioning system (“GPS”) signal, even with a high-quality receiver and antenna, in very low (i.e., “deep fade”) signal environments, such as in urban canyons, underground, and inside steel-reinforced buildings. In the limited situations where a GPS signal can be acquired indoors (e.g., away from steel structures and near windows), far more electrical energy is typically required to acquire the signal and calculate position than is required for “blue sky” conditions, and positional accuracy can be compromised.
Certain algorithmic approaches have been implemented to improve receiver acquisition performance under deep fade conditions, but the improvements are generally limited. There can be many reasons for acquisition and correlation failure on the receiver side in deep fade conditions. The acquisition capability in a well-designed system using high-quality detection algorithms is, however, typically limited by the frequency drift and phase noise generated by the receiver's local oscillator. The uncertainty caused by the local oscillator frequency and phase translates into a receiver bit-error rate, degrading the correlation strength of the coded signal. For weaker satellite signals, the acquisition fails, or a repeatable correlation cannot be found and large position errors are introduced.
In general, phase noise can be viewed as random thermal noise or temperature drift in a resonant element that is converted to random phase shifts or frequency drifts in oscillator output. Very low frequency phase noise (i.e., less than approximately 1 Hz) is usually labeled “frequency drift,” and temperature change in the resonant element is the most significant cause of this frequency drift. For example, if the resonant element is a quartz crystal, rapid thermal fluctuations in the crystal lattice displacement amplitude create rapid phase fluctuations. Thermal fluctuation noise that occurs at harmonics of the resonator oscillation frequency contributes most strongly to the higher frequency phase noise. If the quartz crystal temperature is also slowly changing during operation, the resonant frequency will gradually change, introducing frequency drift.
There has, in general, been little progress with work done on systematic high frequency phase-noise reduction in frequency sources appropriate for portable, small, and low-power applications, such as GPS systems. However, some progress has been made with reducing frequency drift (i.e., low frequency phase noise) in frequency sources for portable receivers. In most high performance GPS systems, quartz crystal-based oscillators called “Temperature Controlled Crystal Oscillators” (TCXOs) use lumped circuit elements to partially compensate for temperature-induced resonant frequency drift in the crystal. The typical TCXO shows about 1/10th the frequency change of an uncompensated crystal over a typical operating temperature range. “Oven-Controlled Crystal Oscillators” (OCXOs) employ physical temperature control of the resonator, and they generally reduce the temperature-variation-induced frequency drift even more than TCXOs. The typical OCXO has from 1/10th to 1/100th the frequency change of a TCXO over a typical operation temperature range.
A battery-powered GPS receiver can be enhanced with a very stable, low frequency drift local-oscillator frequency source. With precise knowledge of time derived from the stable source, the Doppler shift in satellite transmitter frequency may be accurately determined from ephemeris (orbit) data. Consequently, the frequency search space needed to acquire the GPS signal is sharply reduced, leading to significant savings in battery energy. Then, after acquisition is achieved, minimal frequency drift in the local oscillator frequency source during satellite data collection also allows for more rapid and accurate determination of receiver position. This again saves battery energy.
Stable frequency sources also benefit most battery-powered infil digital receivers and exfil transmitters and beacons. For example, the position of a fixed-beacon transmitter deduced by Doppler shift in a passing aircraft typically requires a collection time of seconds to minutes. Consequently, the frequency of the transmitted signal must be stable on this time scale to maintain beacon location accuracy.
In general, conventional OCXOs offer the necessary level of frequency stability, but they generally require relatively high average power, particularly at low environmental temperatures. Moreover, since the OCXO must generally operate at all times, the battery requirements for operating over extended periods of time become prohibitive.
Conventional OCXOs require high average power because of the way in which they are operated. Their crystals (usually quartz) are typically maintained at a temperature of 15° C. to 20° C. higher than the highest temperature rating of the transmitter or receiver unit. Resistive heating elements have been assumed to offer the best combination of low cost and minimum size, so the crystal temperature must be maintained above the highest specified ambient temperature. Since the specified upper range of operating temperature in many applications can be as high as 70° C., the crystal operating temperature is often set at 90° C. or higher. The high crystal operating temperature of the OCXO also requires that the resonant frequency of the crystal stabilize when the OCXO is first powered “on,” i.e., if the OCXO is activated from room temperature in an effort to save power between uses, the temperature and strain fields must generally be allowed to thermally stabilize at the high operating temperature before the resonant frequency is fully stabilized. This stabilization time can be on the order of several minutes, an unacceptably long waiting period for many applications.
A higher-than-ambient-temperature crystal, as used in a conventional OCXO, implies high heat transfer and high continuous energy loss to the environment. This loss may be mitigated by using thermal insulation. However, in applications where the size of the crystal unit is critical, a significant volume of thermal insulation is unacceptable. Consequently, OCXOs are almost never used in miniaturized portable receivers and transmitters where only battery power is available.
Reduced temperature has been shown to reduce high frequency (i.e., greater than approximately 1 Hz) phase noise in quartz, sapphire, and rubidium resonators. The temperature at which high frequency phase noise is reduced significantly depends on the practical details of the resonator material. For example, the level of contaminants and crystal defects in quartz will generally impact the temperature at which the high frequency phase noise is reduced. Cryogenic cooling systems, which are usually mechanical, are often used for cooling crystals in specialized receivers intended for receiving signals from deep-space probes, and this cooling is effective at reducing high frequency phase noise. However, such systems generally consume significant electrical power (i.e., many watts) and take up substantial volume (i.e., many cubic centimeters), which is generally unsuitable for portable, small, and low power applications, such as GPS systems.
Accordingly, needs exist for improved devices, systems, and methods of i) maintaining a constant resonant element temperature, and ii) reducing the temperature of the resonant elements used in portable, small, and low-power applications, such as GPS systems, so as to efficiently reduce frequency drift and higher frequency phase noise contributed by those resonant elements.